The present invention relates to a process for preparing silver powder with a controlled surface area by reduction reaction. More particularly, it relates to a continuous process for preparing silver powder by controlling the reaction time at low temperature and monitoring the rest potential of the reaction solution.
Growth in the electronics industry recently has led to an increased demand for precious metal powders. Silver powder, one of the precious metal powders, has become a very important conductive material in the electronics industry because of its superior conductivity, chemical stability and low price.
The main process for preparing silver powder is by chemical reduction. In carrying out the process, the starting silver nitrate is adjusted to alkaline with ammonium hydroxide, followed by adding a reducing agent, such as hydrazine, formaldehyde, acetaldehyde and reducing sugars to obtain silver precipitate. The reaction proceeds in accordance with the following equation: EQU silver ions+reducing agent.fwdarw.silver particles
The size of the particles thus produced is about 0.5 to 10 .mu.m, and the density is 0.35 to 1.8 g/cm.sup.3. However, the surface area(S.A.) of the produced particles merely ranges from 0.5 to 2 m.sup.2 /g. This is probably because the particles form cakes. The size, density and S.A. of the resulting silver powder are all suitable for use as electronic material. Thus, this technique has become the main process for preparing silver powder in the electronics industry. (Montino et al., U.S. Pat. No. 4,039,317 and Fraioli et al., U.S. Pat. No. 3,966,463)
Taiwan Patent No. 43,382 discloses an improved process for preparing silver powder. The procedures for this process are shown in FIG. 1. This process is characterized in that the reducing agent is first mixed with surfactants during reducing process prior to reacting with a silver ammoniacal solution. By this process, the amount of wastewater is reduced, and the yield of silver powder is increased.
Jost et al. in their U.S. Pat. Nos. 4,456,473 and 44,456,474 have disclosed that by mixing ammonium hydroxide with silver nitrate, then with hydrazine, or mixing ammonium hydroxide with hydrazine, then with silver nitrate, silver powder with a particle size of 3 to 5 .mu.m(the former) or 0.6 to 2.5 .mu.m(the latter) can be obtained as long as the molar ratio is appropriate. However, according to the examples set forth in the two patents, the reaction solution must be sprayed through a nozzle at a high pressure of 5000 psi. This adds to the difficulty of installing the manufacturing equipment and also increases the danger of operating it. Furthermore, the surface area of the silver powder produced by this method is insufficent.
Silver powder for making thick films of conductors must have suitable properties for being mixed with solvents and glass powders, etc. to obtain silver pastes. The properties of silver powder are determined by its surface area, particle size, particle size distribution, tap density and manner of surface treatment. However, as prior methods for producing silver powder all involve batch type chemical reduction processes, the properties of silver powder can only be controlled by varying the concentrations of reactants, the stirring speed of the reactors containing the reactants, and the reaction temperature based on experiential data. Therefore, silver powder with a controlled surface area is often unobtainable. The qualities of electronic products using the silver powders are thus tremendously affected.
To obtain silver powder having a controlled surface area by batch type chemical reduction processes, the concentrations of the reactants must be adjusted precisely, or the produced silver powders must be subjected to post mechanical grinding treatment. According to the methods of U.S. Pat. No. 4,456,473 and U.S. Pat. No. 4,456,474, properties of silver powder produced by reacting hydrazine, ammonium hydroxide and silver nitrate are controlled by changing their concentrations. In the examples, the particle size of the produced powders were respectively controlled to within the range of 3 to 5 .mu.m and 0.6 to 2.5 .mu.m, and the surface areas were controlled to about 2 m.sup.2 /g. However, as the silver powder was produced by batch type reaction, the surface area of silver powder produced by each batch can not be controlled to within a desired range. That is to say, silver powders produced by each batch will have different surface area. Subjecting the produced silver powder to post-grinding treatment can produce silver powder with suitable surface area, however, post-grinding will cause the resultant silver powder to become flake shaped because of its high ductility. This morphology change will restrict its usefulness in the electronics industry.